1. Field of the Invention
The invention generally relates to thermal isolation devices which are used as heat shields when interposed between a temperature sensitive device and a heat source. More particularly, the invention relates to a high efficiency thermal interposer (sometimes referred to hereinafter as a heat shield) for shielding a robot arm (on which high accuracy positioning devices are typically located), from the heat generated by a bonding device, such as a Hot Air Thermode (HAT). HAT devices are used for performing inner and outer lead bonding of surface mounted components to substrates through solder reflow.
2. Description of the Related Art
It is a constant endeavor in the field of heat transfer to provide means for improving the efficiency of the heat transfer process. For example, U.S. Pat. No. 3,732,919, discloses a heat exchanger having numerous solid particles such as coarse sand, gravel or spherical pelletsof concrete packed within a coolant pipe. The claimed effect of such packed particles is to give a coolant moving through the pipe a greater heat exchange surface area across which heat is then communicated from particle to particle and finally to the inner layer of the pipe. The preferred embodiments of the Wilson patent comprise long flat sheets of metal separated only by the contiguous particles. The solid particles both support the sheet metal and transfer the heat to it.
Other patents such as U.S. Pat. Nos. 3,161,478; 3,946,720; 4,593,754 and 4,600,052, also disclose heat exchangers having a pourous innerspace, e.g., spherical balls, imperforate strips, etc., which form a channel for a fluid coolant which removes the heat absorbed by the pourous innerspace. Claimed improvements in the above referenced patents are directed to providing more compact heat exchangers which are light and fit into small spaces; to providing structures through which coolants may be dispersed in an improved manner, etc.
These improved heat exchangers are commonly used to cool, for example, individual electronic components, integrated circuit packages, etc. as typified by the disclosures set forth in U.S. Pat. Nos. 3,327,776; 4,531,146; 4,612,978 and 4,730,665.
A variety of heat exchange techniques have been employed in conjunction with bonding tools typically used in the manufacture and/or repair of semiconductor electronic devices. In U.S. Pat. No. 4,295,596 apparatus and methods are described for bonding an article to a metallized substrate, at least one of which has a predeposited reflowable bonding material (e.g., solder) thereon. The described method comprises the steps of positioning the article on the substrate; directing at least one stream of hot gas towards the substrate; redirecting the gas stream laterally, towards the article, to reflow the material; and resolidifying the material to form a bond between the article and the substrate.
The bonding tool described in the U.S. Pat. No. 4,295,596 patent is fixedly mounted to a support which is mounted for vertical movement. The tool includes a nozzle which is positioned, over the article and substrate, via push rods that are used to overcome a spring force in the body portion of the tool. A plurality of cooling fins on the tool itself removes the tool generated heat.
Another type of bonding tool using different heat exchange methodology is disclosed in U.S. Pat. No. 4,300,715. This tool is described in the context of an assembly system for assembling elements of microcomponent devices like integrated circuits and other electronic devices. The disclosed system operates at temperatures in the approximately 400 degrees C. range, and uses a pneumatically controlled bonding tool to move a thermode into and out of contact with the elements to be bonded. In this system, a substrate support table accurately positions the substrate under the bonding tool.
The system taught in the U.S. Pat. No. 4,300,715 patent uses the substrates as heat sinks when the substrates themselves are not being heated by the tool. The heating tool includes a heating anvil, the tips of which are air cooled.
Neither the U.S. Pat. No. 4,295,596 nor the 4,300,715 patent are concerned with the cooling or shielding of the positioning means used in conjunction with the substrates and/or bonding tools taught in these references.
Hot air thermodes (HAT), such as the device taught in U.S. Pat. No. 4,937,006 (hereby incorporated by reference) provide a low load, fluxless and low cost method of inner and outer lead bonding of surface mount components through solder reflow. These state of the art thermode devices have high power outputs and thus generate a lot more heat in the heat exchanger then is used to heat the reflow gas. Since, because of efficiency and temperature requirements, this heat exchanger is closely positioned to the bonding nozzle, the whole assembly must be mounted on high-accuracy positioners at the end of a robot arm. The stray heat that escapes through the mounting shaft and/or brackets into the positioners causes temperature increase in the robot assembly itself, impacting accurate alignment and operation.
To prevent such temperature increase in the robot arm, a suitable heat shield must be interposed between the robot arm and the HAT device. Such a heat shield must be compact to fit in the limited space available for most pick place attach (PPA) tools, have low mass to limit the inertia of the robot arm assembly and cannot use liquids as a cooling medium to eliminate possible coolant contamination of the components being assembled.
The desired heat shield must be able to withstand the heat generated by state of the art high powered HAT devices. These devices operate at approximatly 700 degrees C. with power levels from 600 to 1500 watts. Also, the heat shield must be effective under circumstances where the bonding nozzle of the HAT device may be located less than 1 inch from the robot arm assembly. The robot arm assembly typically cannot be exposed to more than approximately 100 degrees C. without severely affecting sensitive components located in the assembly. The normal operating temperature for robot arm assemblies of the type contemplated herein is approximately 40 degrees C.
Accordingly, it would be desirable to provide a high efficiently, low mass thermal interposer that can drop temperatures on the order of 700 degrees C. to temperatures on the order of 40 degrees C., over less than approximately one inch.
Furthermore, it would be desirable if such an interposer could be cooled without using liquids to thereby eliminate the possiblity of coolant contamination of the components being assembled.
Still further, it would be desirable if an interposer possessing the above stated characteristics were available for use in conjunction with high powered HAT devices, to enable such devices to be directly attached to heat sensitive PPA assemblies (such as robots) in a safe, reliable and efficient manner.